Method for making an integrated circuit of the surface-mount type and resulting circuit

ABSTRACT

The invention relates to a method for making an integrated circuit ( 40 ) of the surface-mount type the comprising, first of all, manufacture of a package having a rear face and a pin grid array extending under this rear face perpendicular thereto, and a ball ( 44 ) of low melting point alloy is then formed at the end of each pin surrounding this end and soldered thereto. 
     The invention also relates to an integrated circuit ( 40 ) of the surface-mount type, comprising a package having a rear face and a pin grid array, of a cross section roughly constant along the pin ( 42 ), extending under the rear face perpendicular thereto. A ball ( 44 ) of low melting point alloy is soldered to the end of each pin ( 42 ) surrounding this end.

The invention relates to a method for making an integrated circuit forthe surface-mounting of electronic boards. More specifically, theinvention relates to the production of electrical connections betweenthe package and the printed circuit of the electronic board.

The increasing complexity of integrated electronic circuits, and theincrease in the number of semiconductors that can be integrated into theself-same chip is leading to an increase in the number and density ofintegrated circuit output connections. Integrated circuit surfaceconnection technology is ever evolving along these lines.

Certain integrated circuit packages of the prior art have rows of leadsarranged with a uniform pitch, perpendicular to the edges of theintegrated circuit, the end of each of the leads being bent at 90° withrespect to a rear face of the integrated circuit, this rear face facinga face on which the electronic board to which the integrated circuitwill be soldered is mounted. Bending the leads at 90° allows them to besoldered to the mounting face of the board. This type of connection,commonly known as dual-in-line or DIL, has the disadvantage of beingbulky and of allowing only the production of packages with a limitednumber of outputs.

Connections using an array of pins or connection technologies known aspin grid arrays or PGAs have long been used in microelectronics. Therear face of the printed circuit package produced using this PGAtechnology is equipped with the array of pins which are situated underthis rear face and are perpendicular to this face. In this connectiontechnology, the pins have to be inserted into the electronic board, andthis uses up a significant amount of space.

In the technology of surface-mount integrated circuits (SMCs) J-shapedleads or J-leads bent on the edges of the integrated circuit package aresoldered to the printed circuit.

In the latest generations of integrated circuit packages, the connectionbetween the integrated circuit and the printed circuit is performed byan array of balls made of tin/lead alloy. This technology, known as BallGrid Array or BGA allows a very dense packing of connections which isvertically shorter, thus shortening the length of the connectionsbetween the integrated circuit and the electronic board. One of theadvantages of this technology is its better frequency behavior.

At the outset, the BGA technology was developed in order to obtain ahigh number of output pins on small footprint packages, but these daysthis technology is used even for small chips with a small number ofoutputs (for example 40-pin memories).

FIG. 1 depicts a sectioned part view of a ceramic package 10 of anintegrated circuit comprising a chip 12 mounted on the package. FIG. 1shows details of one of the output connections of the package usingconnection technology of the ball grid array or BGA type.

In this example of FIG. 1, the chip 12 is inverted so as to show itselectrical access points situated on its active face, facing thesubstrate for interconnection of the package 10, so as to be soldereddirectly to the substrate of the package. In other embodiments, the chip(not inverted) may be soldered to the package by wire connections.

The chip 12 is connected by soldered connections 14 to internalconnections 16 of the package 10. The internal connections 16 areperformed on the outside of the package, via metal pads 18 situated onthe same side as a rear face 19 of the package 10. Balls 20 made oftin/lead alloy intended to be soldered to the printed circuit (notdepicted in the figure) are soldered to the metal pads 18.

This type of BGA connection in FIG. 1 has significant disadvantages.Specifically, when the substrates of the package and that of the boardon which the package is mounted have different coefficients ofexpansion, for example in the case of a ceramic package and a board madeof epoxy resin, the differential expansion between these two substratesas the temperature varies causes the solder balls 20 to break. Theheight of the balls made of tin/lead alloy is not enough to alleviatethis problem and the soldered joints break relatively quickly withthermal cycles.

The breakages of the solder balls may occur, on the one hand, at thetime of the soldering of the package to the electronic board because ofthe thermal shock produced as the ball 20 melts then cools relativelyquickly and, on the other hand, during operation of the integratedcircuit when there are variations in the ambient temperature, it beingpossible for this variation in ambient temperature to be very great andvery rapid (−55° C. to +150° C.) in the case of military applications.

Another disadvantage with this connection technology of the ball gridarray or BGA type is that the package is difficult to recover in orderto replace an expensive component in the event of breakdown.

Package manufacturers are proposing solutions for avoiding the breakageof the solder balls of the packages soldered to a board.

FIG. 2 shows one solution which consists in using an insert 22consisting essentially of a ceramic plate 24 with holes 26. The holes 26are arranged with the same pitch as the metal pads 18 of the connectionsof the package.

Before the insert 22 is mounted on the package, an elongate-shapedsolder ball 28 is produced in each of the holes 26, the balls 28protruding from each side of the ceramic plate 24.

The ceramic plate with the elongate balls 28 is arranged, on the side ofone of its faces, under the rear face 19 of the package 10. The ends ofthe balls protruding from the holes on one and the same side of theceramic plate are soldered to the respective metal pads 18 of thepackage connections.

The other ends of the balls, protruding from the other side of theceramic plate, which are intended to be soldered to an electronic board,are taller than the ends of the balls soldered to the package. Thethermal stresses exerted on the balls of the package equipped with theinsert once soldered to the board are thus spread over the periphery ofthe ball.

When it is desirable to have a greater ball height, the balls then beingin the form of columns, so as to withstand greater thermal stresses, itis necessary to use an appropriate mold.

Another solution for avoiding the breakage of the solder balls isdepicted in FIG. 3.

A package 30 is hollowed out with semispherical cavities 32 at thepoints of connection of the package. Into each of the cavities 32covered with a layer of metal 34 is inserted a spherical solder ball 36.In this type of connection involving a cavity, the thermal stresses arespread over a larger area without having to use an insert that requiresa mold in order to produce it.

In the case of ceramic packages, the balls used for the solder areusually made of an alloy containing 93% lead and 7% tin and which arenot readily moldable. In the case of plastic packages, the alloycontains 63% lead and 37% tin. It would not be possible to use an alloycontaining 63% lead and 37% tin in the case of ceramic packages becausethat would lead to excessive crushing of the balls when soldering thepackage to the board.

The current solutions to the problem of the breakage of the solderedjoints are not satisfactory in terms of cost or performance. Inparticular, the insert is an expensive solution. The use of the columnslimits the problem of ball breakage but this solution, in militaryapplications, is nonetheless technically inadequate.

In order to alleviate the integrated circuit package connection problemsof the prior art, the invention proposes a method for making anintegrated circuit of the surface-mount type, comprising, first of all,the manufacture of a package having a rear face and a pin grid array,the array extending under this rear face perpendicular thereto and thenthe formation at the end of each pin of a ball of low melting pointalloy surrounding this end and soldered thereto, characterized in that,in order to form the ball at the end of each pin:

-   -   a cellular plate the working cells of which are distributed at        the same pitch as the pins of the package of the integrated        circuit are filled with a solder cream formed from the material        of the alloy;    -   the pins of the pin grid array of the integrated circuit are        inserted into the cells containing the solder cream, the        integrated circuit being on top of the cellular plate;    -   the cellular plate is heated until the alloy melts;    -   the cellular plate is cooled very quickly so as not to allow the        liquid alloy enough time to wick up along the pins;    -   the integrated circuit and the cellular plate secured to the        circuit by the solidified alloy in the cells is then inverted so        as to place the cellular plate on top of the integrated circuit,        the integrated circuit being suspended from the cellular plate        by its pins held in the alloy solidified in the cells;    -   the cellular plate is heated until the solder in the cells        melts, causing the integrated circuit to separate from the        cellular plate under gravity and causing solder balls to form at        the ends of the pins, the alloy having wetted the pins        sufficiently for it to remain attached to the pins and        solidifying quickly in the form of balls as soon as the        integrated circuit separates from the cellular plate, the alloy        not having time to spread out along the pins.

The invention also proposes an integrated circuit of the surface-mounttype achieved by the method of manufacture according to the invention,comprising a package having a rear face and a pin grid array extendingunder the rear face perpendicular thereto, characterized in that the endof each pin has a ball of low melting point alloy soldered to the end ofeach pin surrounding this end.

In certain embodiments of the integrated circuit according to theinvention, the length of pin end surrounded by the ball is roughly equalto the diameter of the ball. This results in particular embodiments suchthat, in a first embodiment, the length of pin end surrounded by theball is equal to the length of pin protruding from the rear face of thepackage and, in a second particular embodiment, the length of pinprotruding from the rear face of the package is greater than thediameter of the ball.

In other embodiments of the integrated circuit according to theinvention, the length of pin end surrounded by the ball is less than thediameter of the ball.

The invention may apply to integrated circuits produced with eitherceramic packages or plastic packages.

The method of manufacture according to the invention is particularlywell suited to integrated circuits comprising pins the cross section ofwhich is roughly constant along the pin, particularly when the pins aresmooth, which they generally are.

Other features and advantages of the invention will become apparent fromreading the detailed description given thereof with reference to theappended drawings in which:

FIG. 1, already described, depicts a sectioned part view of a ceramicpackage of the prior art;

FIG. 2, already described, shows a sectioned view of the package of FIG.1, using an insert;

FIG. 3, already described, shows a sectioned view of a package of theprior art comprising solder balls arranged in cavities of the package;

FIG. 4 depicts an integrated circuit according to the inventioncomprising pins with solder balls.

FIGS. 5 a, 5 b, 5 c and 5 d depict detailed views of various embodimentsof pins of the integrated circuit according to the invention.

FIG. 6 shows a detailed view of a pin of the integrated circuit of FIG.4 soldered to an electronic board.

FIGS. 7, 8, 9 and 10 show various steps in the manufacture of theintegrated circuit according to the invention.

FIG. 4 shows an integrated circuit 40 according to the inventioncomprising smooth connection pins 42 of a cross section roughly constantalong the pin, each of the pins having, at its end, a ball 44 of soldersurrounding this end. The pins are made of ferronickel which is platedwith nickel then plated with gold, in the case of ceramic packages, andwith Kovar in the case of plastic packages.

The integrated circuit 40 can be produced with various configurationsregarding the length of the pins and the positions of the balls at theends of the pins. FIGS. 5 a to 5 d show details of one of the pins ofthe integrated circuit with various configurations.

FIG. 5 a shows details of one of the pins of an embodiment of theintegrated circuit 40 equipped with pins of a length Lb greater than thediameter of the ball 44 soldered to its end. Considering the part of thepin Le surrounded by the ball, that we shall hereinafter term the pinend, in the embodiment of FIG. 5 a, the length of the pin end is roughlyidentical to the diameter D of the ball 44. This embodiment has theadvantage of not causing the solder balls to become crushed as theymelt, as the integrated circuit is transferred onto the board. Whathappens is that the balls are internally reinforced by their respectivepins which pass right through their diameter D.

In another embodiment, the integrated circuit is equipped with shortpins 46, the length of which is roughly equal to the diameter of thesolder ball 44, the ball surrounding the entirety of the pin. FIG. 5 bdepicts a view of one of the pins of such an integrated circuit. Thisconfiguration with short pins allows the package to be soldered veryclose to the printed circuit of the board while at the same timeavoiding the crushing of the solder ball under the weight of theintegrated circuit as it is transferred onto the electronic board, theball being reinforced, as in the embodiment of FIG. 5 a, over theentirety of its diameter D.

In other embodiments of the integrated circuit, the length of the pinend Le, surrounded by the ball 44, is less than the diameter of theball.

FIG. 5 c shows details of one configuration of the pin 42 of FIG. 5 a,in which configuration the length of the pin end Le surrounded by theball 44 is less than the diameter D of the ball.

FIG. 5 d shows details of a short pin 48 in a different configurationfrom that of FIG. 5 b. In the configuration of FIG. 5 d, the length Lbof the pin is less than the diameter D of the ball, all of the pin 48being surrounded by the ball 44.

The balls 44 of the various configurations of the pins may be producedfrom an alloy containing 63% lead and 37% tin which has only advantages,namely:

-   -   a very low melting point;    -   wettability;    -   the fact that it is an alloy identical to the one used on the        printed circuit; or any other alloy suited to the transfer of        components onto the surface.

FIG. 6 shows details of the pin 42 of FIG. 5 a after the integratedcircuit 40 has been soldered to an electronic board 50 having mountingmetallizations 52. The metallizations have, on their surface, solder 54made of an alloy generally containing 63% lead and 37% tin.

In the embodiments of FIGS. 5 a and 5 b, at the time that the integratedcircuit is mounted on the board, the solder 54 on the metallization 52rises up toward the end of the pin 42. It is therefore not necessary tocrush the solder ball 44 in order to solder the pins to the board.

Another advantage of this invention lies in the fact that even if thesolder between the pin and the metallization of the board breaks,electrical contact will be afforded by the pin in contact with theplating. Indeed, the pin which is soft-soldered to the ceramic packageis a very secure connection.

FIGS. 7, 8, 9 and 10 show the main phases of the method of manufactureaccording to the invention for making the integrated circuit of FIG. 4.

In a first phase shown in FIG. 7, an integrated circuit 60 of thesurface-mountable type is made, this involving first of all making apackage having a rear face 62 and a pin grid array of smooth pins 64extending under this rear face perpendicular thereto.

The length and the diameter of the pins 64 will be chosen according tothe constraints of the application and the reliability requirements.

The ends of the pins 64 may be tinned or dipped in flux or any othersuitable chemical composition depending on the surface finish of theprinted circuit 60 using PGA technology.

In the first phase depicted in FIG. 7, a cellular plate 68, the cells 70of which are distributed on the plate with the same pitch as the pins 64of the package of the printed circuit 60, is filled with a solder cream66 made from the material of the alloy and solvents.

In a second phase depicted in FIG. 8, the ends of the pins 64 of the pingrid array of the integrated circuit 60 are inserted into the cells 70containing the solder cream 66, the integrated circuit being on top ofthe cellular plate, then the alloy is heated by passing the cellularplate over a hot plate until the alloy melts and the cellular plate iscooled very quickly so as not to give the liquid alloy time to wick upalong the pins causing the alloy to solidify in the cells.

In a third phase, depicted in FIG. 9, the integrated circuit 60 and thecellular plate 68 secured to the integrated circuit by the alloysolidified in the cells is inverted so as to place the cellular plate ontop of the integrated circuit. In this phase, the integrated circuit issuspended from the cellular plate by its pins held in the alloysolidified in the cells 70 of the plate.

In a fourth phase depicted in FIG. 10, the cellular plate is heateduntil the alloy in the cells melts, causing the integrated circuit toseparate from the cellular plate under gravity and causing solder balls72 to form at the ends of the pins. The alloy has wetted the pinssufficiently to remain attached to the pins, solidifying quickly intothe form of balls, as soon as the integrated circuit separates from thecellular plate, the alloy not having time to spread out along the pins.In this fourth phase, the integrated circuit, as the alloy melts againin the cells, drops under its own weight onto a receptacle 80 providedfor this purpose, situated under the integrated circuit (see FIG. 10).The distance between the integrated circuit and the receptacle is atminimum equal to the depth of the cells in the cellular plate.

It must be pointed out that, in the second phase of the manufacturingmethod depicted in FIG. 8, the order of the two operations consisting ininserting the ends of the pins 64 of the integrated circuit 60 into thecells 70 containing the solder cream 66 and in heating the alloy bypassing the cellular plate over a hot plate until the alloy melts isunimportant and can be reversed without changing the result. It is thusalso possible first of all to melt the solder cream in the cells then toinsert the ends of the pins 64 of the integrated 60 into the cells.

The solder paste, formed for example from an alloy material containing63% lead and 37% tin, may be made from any other alloy suited to thesurface mounting of components.

This technology of mounting solder balls at the ends of the pins is veryinexpensive and does not require a machine for positioning preformedballs.

The cellular plate 68 is made either of graphite or of titanium or ofsome other material suited to reflow. The solder cream usually contains50% tin/lead alloy and 50% fluid.

In general, in the manufacturing method, the ends of the pins areinserted into the cellular plate as far as the closed ends of the cells,so as to obtain the pins of FIGS. 5 a and 5 b, the solder ballssurrounding the end of the pin over a length roughly equal to thediameter D of the ball.

In an alternative form of the method for making the integrated circuit,the molten alloy in the cells of the cellular plate is obtained byplacing sized balls of alloy in a cellular plate, the balls are meltedand the pins of the pin grid array of the integrated circuit areinserted into the cells containing the molten alloy, the integratedcircuit being on top of the cellular plate, the next steps being thesame as in the method described above, namely:

-   -   the cellular plate is cooled very quickly so as not to allow the        liquid alloy enough time to wick up along the connections;    -   the integrated circuit and the cellular plate secured to the        circuit by the solidified alloy in the cells is then inverted so        as to place the cellular plate on top of the integrated circuit,        the integrated circuit being suspended from the cellular plate        by its pins held in the alloy solidified in the cells;    -   the cellular plate is heated until the solder in the cells        melts, causing the integrated circuit to separate from the        cellular plate under gravity and causing solder balls 72 to form        at the ends of the pins.

It must be pointed out that, in this alternative form of the method ofmanufacture, the order of the two operations consisting in melting theballs placed in the cells and in inserting the pins of the pin gridarray of the integrated circuit into the cells containing the moltenalloy is unimportant and can therefore be reversed without changing theresult. It is thus also possible to insert the pins of the pin gridarray of the integrated circuit into the cells containing the sizedballs, then for the balls to be melted.

The method of manufacture according to the invention converts anintegrated circuit having connectors of the PGA type into BPGA, namelyball pin grid array, technology. Once the integrated circuit has beenproduced, it is mounted on its electronic board using the same method asa conventional BGA.

The integrated circuit according to the invention makes it possible toobtain very densely packed connections while at the same timeguaranteeing better reliability, ease of repair of the circuit and thepossibility of extracting heat from between the connected substrates.

1. A method for making an integrated circuit of the surface-mount type,comprising, first of all, the manufacture of a package having a rearface and a pin grid array, the array extending under this rear faceperpendicular thereto and then the formation at the end of each pin of aball of low melting point alloy surrounding this end and solderedthereto, wherein, in order to form the ball at the end of each pin:working cells of a cellular plate are distributed at the same pitch asthe pins of the package of the integrated circuit are filled with asolder cream formed from the material of the alloy; the pins of the pingrid array of the integrated circuit are inserted into the cellsincluding the solder cream, the integrated circuit being on top of thecellular plate; the cellular plate is heated until the alloy melts; thecellular plate is cooled very quickly so as not to allow the liquidalloy enough time to wick up along the pins; the integrated circuit andthe cellular plate secured to the pins of the circuit by the solidifiedalloy in the cells is then inverted so as to place the cellular plate ontop of the integrated circuit, the integrated circuit being suspendedfrom the cellular plate by its pins held in the alloy solidified in thecells; the cellular plate is heated until the solder in the cells melts,causing the integrated circuit to separate from the cellular plate undergravity and causing solder balls to form at the ends of the pins, thealloy having wetted the pins sufficiently for it to remain attached tothe pins and solidifying quickly in the form of balls as soon as theintegrated circuit separates from the cellular plate, the alloy nothaving time to spread out along the pins.
 2. The method for making anintegrated circuit of the surface-mount type as claimed in claim 1,wherein the ends of the pins are inserted in the cellular plate as faras the closed end of the cells.
 3. The method for making an integratedcircuit of the surface-mount type as claimed in claim 1, wherein thecellular plate is made either of graphite or of some other materialsuited to reflow.
 4. The method for making an integrated circuit of thesurface-mount type as claimed in claim 1, wherein the length of pin endLe surrounded by the ball is roughly equal to the diameter D of theball.
 5. The method for making an integrated circuit of thesurface-mount type as claimed in claim 4, wherein the length of pin endLe surrounded by the ball is equal to the length of pin Lb protrudingfrom the rear face of the package.
 6. The method for making anintegrated circuit of the surface-mount type as claimed in claim 1,wherein the length of pin protruding from the rear face of the packageis greater than the diameter of the ball.
 7. The method for making anintegrated circuit of the surface-mount type as claimed in claim 1,wherein the length of pin end Le surrounded by the ball is less than thediameter of the ball.
 8. The method for making an integrated circuit ofthe surface-mount type as claimed in claim 1, wherein the ball is atin/lead alloy.
 9. The method for making an integrated circuit of thesurface-mount type, comprising, first of all, the manufacture of apackage having a rear face and a pin grid array, the array extendingunder this rear face perpendicular thereto and then the formation at theend of each pin of a ball of low melting point alloy surrounding thisend and soldered thereto, wherein, in order to form the ball at the endof each pin: sized balls of alloy are placed in the cells a cellularplate, the cells of which are set out at the same pitch as the pins ofthe package of the integrated circuit; the balls are melted; the pins ofthe pin grid array of the integrated circuit are inserted into the cellscontaining the molten alloy, the integrated circuit being on top of thecellular plate; the cellular plate is cooled very quickly so as not toallow the liquid alloy enough time to wick up along the connections; theintegrated circuit and the cellular plate secured to the pins of thecircuit by the solidified alloy in the cells is then inverted so as toplace the cellular plate on top of the integrated circuit, theintegrated circuit being suspended from the cellular plate by its pinsheld in the alloy solidified in the cells; the cellular plate is heateduntil the solder in the cells melts, causing the integrated circuit toseparate from the cellular plate under gravity and causing solder ballsto form at the ends of the pins, the alloy having wetted the pinssufficiently for it to remain attached to the pins and solidifyingquickly in the form of balls as soon as the integrated circuit separatesfrom the cellular plate, the alloy not having time to spread out alongthe pins.
 10. The method for making an integrated circuit of thesurface-mount type as claimed in claim 1, wherein the pins have a crosssection which is roughly constant along the pin.
 11. The method formaking an integrated circuit of the surface-mount type as claimed inclaim 1, wherein the pins are smooth.
 12. The method for making anintegrated circuit of the surface-mount type as claimed in claim 9,wherein the pins have a cross section which is roughly constant alongthe pin.
 13. The method for making an integrated circuit of thesurface-mount type as claimed in claim 9, wherein the pins are smooth.14. The method for making an integrated circuit of the surface-mounttype as claimed in claim 9, wherein the length of pin end Le surroundedby the ball is roughly equal to the diameter D of the ball.
 15. Themethod for making an integrated circuit of the surface-mount type asclaimed in claim 14, wherein the length of pin end Le surrounded by theball is equal to the length of pin Lb protruding from the rear face ofthe package.
 16. The method for making an integrated circuit of thesurface-mount type as claimed in claim 9, wherein the length of pinprotruding from the rear face of the package is greater than thediameter of the ball.
 17. The method for making an integrated circuit ofthe surface-mount type as claimed in claim 9, wherein the length of pinend Le surrounded by the ball is less than the diameter of the ball. 18.The method for making an integrated circuit of the surface-mount type asclaimed in claim 9, wherein the ball is a tin/lead alloy.